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548                            Compressed Air.

stood that the employment of fuel in a re-heater is attended with
some six times the economy of the same fuel used in a steam
boiler furnace. Figs. 563-4 shew at B an electrical re-heater
formed of resistance coils in the circuit of the dynamo A, and at
G a stove re-heater through which the air pipe passes. Still
another saving is obtained by air injection. As heat is nothing
but a form of work, it may be made to do work as soon as
generated, instead of being allowed to dissipate. In Fig. 564
this is done by allowing the hot air to pass from the receiver D
through the injector nozzle F, and thus an additional quantity of
air is drawn into the cold receiver E to fill up the loss caused by
shrinkage during cooling. The air being compressed to 100 Ibs.
at a temperature of 484, is reduced to 50 Ibs. in E, with a tem-
perature of 201 y but the gain is certain, for the heat has been
made to do work.

Much mechanical improvement has been introduced in the
compressors, such as the use of lever-lifted valves instead of air-
moved flaps, avoiding wire-drawing. Clearance spaces have been
much reduced, and the mains increased so as to bring the air
velocity below 30 ft. per sec. Referring now to Fig. 561, the
cylinders A A are compound high and low pressure, and the air
enters first the suction valves F F of the cylinder B. Leaving by the
valves E E, it passes by b to the surface condenser D, and then to
the second cylinder c, which it enters by H H and leaves by G G.
Finally it passes by pipe M to the storage receiver. The valves
are lifted by levers P, moved by cams N on shafts d d.

The Paris Compressed Air Company delivers about 8000 H.P.
from two central stations, through thirty-five miles of piping, the
further motor being 4! miles away. Prof. Kennedy measured the
efficiencies in 1889, and found that for one I.H.P. in central
engine, the customer received -39 I.H.P. with cold air and '47
with air re-heated just before entering his motor. With two-
stage compression and other improvements, Frangois shewed
in 1891 that a total efficiency of 46 could be reached with
cold air, '65 with hot air, and *8 if the hot air was sprayed
with water; which results have since been approached. This
assumes efficiencies of compressor, main, and motor at "9, '96, and
93 respectively.